In recent years there has been a requirement for a magnetic recording medium with higher recording density and lower noise. Various conventional compositions and structures of a magnetic layer and materials for a non-magnetic under-layer and a seed layer have been proposed. In particular, a magnetic layer called the granular magnetic layer has been proposed having a structure in which a ferromagnetic grain is surrounded by non-magnetic non-metallic substance, such as oxide or nitride.
Japanese Unexamined Patent Application Publication No. H8-255342, for example, discloses attaining low noise by forming a granular recording layer in which ferromagnetic grains are dispersed in a non-magnetic film. This is accomplished by a method comprising steps of sequentially depositing a non-magnetic film, a ferromagnetic film and a non-magnetic film on a non-magnetic substrate, and heat-treating the laminate. For this type of conventional magnetic layer, cobalt or an alloy containing cobalt as a main component is used. A metal, oxide, nitride, carbon or carbide is used for the non-magnetic film.
U.S. Pat. No. 5,679,473 discloses that a granular recording film, in which each magnetic grain is surrounded by a non-magnetic oxide and separated with each other, can be formed by means of RF (radio frequency) sputtering using a CoNiPt target added with an oxide, such as SiO2. Low noise is achieved by such a conventional recording film.
The low noise achieved in the above recording film is considered to be achieved by the following reason. Since each of the magnetic grains in this granular magnetic film is physically separated by a grain boundary of non-magnetic non-metallic phase, magnetic interaction between the magnetic grains is reduced and formation of the magnetic domain wall with a zigzag shape at the transition region of a recording bit is suppressed.
Noises of a recording medium are caused by fluctuation of magnetization due to magnetic interaction between magnetic grains that constitute the medium, and the size of the grain. In order to maintain high SNR keeping up with enhancement of the recording density, it is necessary to hold the number of magnetic grains per bit cell greater than a certain value. In other words, minimization of the size of the magnetic grain is required. However, in the situation where large exchange interaction arises between the magnetic grains, the minimization of magnetic grains frequently does not necessarily mean minimization of unit of reversed magnetization. Therefore, it is also necessary to suppress the exchange interaction between the grains for minimizing the unit of reversed magnetization itself that is represented by an activation magnetic moment. Further in the minimization, the magnetic grain itself must have a relatively large value of energy of magnetic anisotropy so that a superparamagnetic state does not occur and the magnetic characteristic essential for high resolution recording, that is, a large Hc/Mrt value, can be obtained. The objective aimed at by the granular structure, in which magnetic grains having high energy of magnetic anisotropy are dispersed in a non-magnetic matrix, is that the above-described rigorous requirements are met for attaining high SNR.
In the conventionally used Co—Cr alloy magnetic film, chromium is segregated from a cobalt alloy magnetic grain towards a grain boundary, so as to reduce magnetic interaction between the magnetic grains. On the other hand, in the granular magnetic layer, the grain boundary phase is composed of a non-magnetic non-metallic substance, which segregates easier than the conventional chromium. Consequently, isolation of magnetic grains is easily enhanced. In the conventional Co—Cr alloy magnetic layer, heating the substrate up to 200° C. is essential for sufficient segregation of chromium when laminating the layer. The granular magnetic layer has the advantage that the non-magnetic non-metallic substance segregates even in lamination without heating.
However, a magnetic recording medium having a granular magnetic film requires addition of relatively large amount of platinum to the cobalt alloy to attain desired magnetic characteristic, in particular, high coercive force Hc. To achieve a coercive force of 2,800 Oe in a granular magnetic film, as high as 16 at % of platinum is commonly needed, while in the conventional CoCr alloy magnetic film, only 8 at % of platinum is required for obtaining the same value of the coercive force Hc. With the growing density of magnetic recording in recent years, very high coercive force of higher than 3,200 Oe is becoming necessary. As a result, the granular magnetic film that requires large amount of expensive platinum has brought about a problem of rising of manufacturing cost. In addition, more reduction of the media noise is demanded accompanying with enhancement of the recording density.
Moreover, with respect to crystal growth at a low thickness stage, that is an initial growth stage, the granular magnetic layer is disordered and a clear granular structure is not formed. This situation is the main cause of deterioration in magnetic characteristics and electromagnetic conversion characteristics in the low Br δ region, Br δ being a product of remanent magnetic flux density and film thickness. In the future trend for the magnetic layer to become thinner, accompanied by a higher recording density, this deterioration of the magnetic characteristics and the electromagnetic conversion characteristics at the initial growth stage of the granular magnetic layer are difficult problems to solve.
Although the non-magnetic non-metallic substance in the granular magnetic layer on a substrate segregates even in unheated lamination, in-plane orientation of magnetization in the magnetic layer is difficult to attain. An isotropic or random orientation medium is liable to be formed.